Inspection of tubes or other conduits for defects or wear within the walls is a necessary and time-comsuming task for owners and operators of processes and equipment wherein high pressure fluid is transferred. Although many techniques have been developed and used in the prior art for inspecting newly manufactured tubing, it is the inspection of installed tubing which presents the greatest challenge to those in the industry. This is due to two factors: the difficulty in accessing sections of the installed tubing which may be within a tube bank or pressure vessel, as well as the possibility that significant structural degradation is likely to have occurred after an extended period of operation under conditions of high pressure, high temperature, and/or corrosive environment.
One such application wherein inspection of the structural integrity of an installed tube is critical is in the steam generators of a nuclear powered electric generating plant. In such a plant, the heated primary coolant flows through a plurality of inverted U-shaped tubes immersed in water from which steam is generated.
As it is typical in such an application for the primary side coolant pressure to be higher than that of the secondary, steam producing side, it is apparent that a failure of a steam generator tube will cause a leakage from the primary coolant into the secondary system.
The occurrence of such leakage is not unusual in such units, especially after extended periods of operation. For this reason, inspection of the individual steam generator U tubes is performed at regular intervals while the plant is shut down for service. An effective test program will not only locate specific failures that have already occurred in the steam generator tubing, but also attempt to identify the specific locations where a failure may be likely to soon occur. Thus, a high accuracy and sensitivity in the testing apparatus is a very desirable feature and has been the goal of the test apparatus heretofore in use and well known in the prior art.
One method of sensing anomalies in a metal structure utilizes the electrical eddy currents set up when a magnetic field and a conductive metal structure are moved relative to each other. The electrical eddy currents induced by such movements may be monitored by a magnetic field sensor and the results analyzed to determine the condition of the inspected metal structure. To insure repeatability and accuracy, this method requires that the current inducing and sensing coils of the probe apparatus be maintained at a close and constant spacing to the surface of the metal being analyzed. For eddy current probes typically used in tubular heat exchangers as discussed hereinabove, the eddy current probe is placed within the interior of the tube being examined and moved longitudinally therethrough.
For tubing with a constant internal diameter, it is relatively straightforward to design an eddy current probe such as that shown in U.S. Pat. No. 4,438,399 to Schnabl et al. The coils of the probe may be maintained at a known spacing from the interior surface of the tube giving accurate and repeatable measurements. However, probes known in the prior art are not suited for use in conduits wherein the internal diameter may not be substantially constant along the length of the tube, as the probe must be designed to traverse the narrowest part of the conduit and may thus be loose or otherwise unsupported laterally in other, larger interior segments.
Situations such as this occur in the case of a previously failed nuclear steam generator tube which has been repaired by welding an internal sleeve over the site of the tube failure. Such internal sleeving results in a segment of the steam generator tube which is of significantly smaller internal diameter than that of the remainder of that particular tube but is not so restrictive as to significantly impair the flow of primary fluid therethrough. Tubes having sleeved sections still require the same careful inspection as do other, intact tubes, but have proved a problem for the probes of the prior art which are unable to both traverse the reduced diameter sleeved segment and give accurate and repeatable measurements of the tube condition in the remainder of the tube.
With the sleeving of failed tubes currently presenting an economically and technically beneficial alternative to the former practice of simply plugging off the failed tube in a nuclear steam generator, the need for an inspection apparatus which meets the rigorous inspection standards of the industry is readily apparent to those skilled in the art.